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Co-Formation of the Disc and the Stellar Halo MNRAS 000, 000–000 (0000) Preprint 13 February 2018 Compiled using MNRAS LATEX style file v3.0 Co-formation of the disc and the stellar halo⋆ V. Belokurov1,2†, D. Erkal1,3, N.W. Evans1, S.E. Koposov1,4 and A.J. Deason5 1Institute of Astronomy, Madingley Rd, Cambridge, CB3 0HA 2Center for Computational Astrophysics, Flatiron Institute, 162 5th Avenue, New York, NY 10010, USA 3Department of Physics, University of Surrey, Guildford GU2 7XH, UK 4Department of Physics, McWilliams Center for Cosmology, Carnegie Mellon University, 5000 Forbes Avenue, Pittsburgh, PA 15213, USA 5Institute for Computational Cosmology, Department of Physics, University of Durham, South Road, Durham DH1 3LE, UK 13 February 2018 ABSTRACT Using a large sample of Main Sequence stars with 7-D measurements supplied by Gaia and SDSS, we study the kinematic properties of the local (within ∼10 kpc from the Sun) stel- lar halo. We demonstrate that the halo’s velocity ellipsoid evolves strongly with metallic- ity. At the low [Fe/H] end, the orbital anisotropy (the amount of motion in the radial di- rection compared to the tangential one) is mildly radial with 0.2 <β < 0.4. However, for stars with [Fe/H]> −1.7 we measure extreme values of β ∼ 0.9. Across the metal- licity range considered, i.e. −3 <[Fe/H]−1, the stellar halo’s spin is minimal, at the level of −1 20 < v¯θ(kms ) < 30. Using a suite of cosmological zoom-in simulations of halo formation, we deduce that the observed acute anisotropy is inconsistent with the continuous accretion of dwarf satellites. Instead, we argue, the stellar debris in the inner halo were deposited in a 10 major accretion event by a satellite with Mvir > 10 M⊙ around the epoch of the Galactic disc formation, i.e. between 8 and 11 Gyr ago. The radical halo anisotropy is the result of the dramatic radialisation of the massive progenitor’s orbit, amplified by the action of the growing disc. Key words: Milky Way – galaxies: dwarf – galaxies: structure – Local Group – stars 1 INTRODUCTION the Galactic disc and the stellar halo can be found in Eggen et al. (1962). Based on the strong apparent correlation between the Stars in the Galactic halo formed earlier compared to those in the metallicity of stars in the Solar neighborhood and the eccentricity disc. However, the epoch of the halo assembly, i.e. the time at which of their orbits, the authors conclude that the metal-deficient stars these stars were deposited into the Milky Way, remains blurry. The formed first in an approximately spherical configuration. This orig- onset of star-formation in the Galactic disc is bracketed to have hap- inal gas cloud then collapsed in a nearly instantaneous fashion, pened between ∼8 and ∼11 Gyr ago, based on white dwarf cooling giving birth to the subsequent generations of stars. The contrac- ages (see Oswalt et al. 1996; Leggett et al. 1998; Knox et al. 1999; tion of the Galaxy not only yielded a population of younger stars Kilic et al. 2017), isochrone modelling (e.g. Haywood et al. 2013; on nearly circular orbits, but also profoundly affected the orbital Martig et al. 2016) and nucleocosmochronology (del Peloso et al. shapes of the old stellar halo. As Eggen et al. (1962) demonstrate, arXiv:1802.03414v1 [astro-ph.GA] 9 Feb 2018 2005). The birth times of the halo’s stellar populations can be es- driven by the collapse, the orbits of the halo stars ought to become timated following the same procedures as for the disc. For exam- highly eccentric, thus explaining the chemo-kinematic properties ple, it has been determined that the halo’s globular clusters range of their stellar sample. Later studies (see e.g. Norris et al. 1985; in age from ∼10 to ∼13 Gyr depending on metallicity (see e.g. Beers & Sommer-Larsen 1995; Carney et al. 1996; Chiba & Yoshii Hansen et al. 2002, 2007; VandenBerg et al. 2013). Most dwarf 1998) revealed that the claimed correlation between the eccentricity spheroidals host stars nearly as old as the Universe itself (see and metallicity might have been caused by a selection bias affect- Tolstoy et al. 2009), while many ultra-faint dwarfs appear to have ing the dataset considered by Eggen et al. (1962). Notwithstanding formed the bulk of their stellar populations only slightly less than the concerns pertaining to the observational properties of their data, a Hubble time ago (see e.g. Belokurov et al. 2007; Brown et al. the theoretical insights into the dynamics of the young Galaxy ex- 2014). In the field, the halo stars have ages in the range of 10-12 posed by Eggen et al. (1962) are as illuminating today as they were Gyr (Jofr´e& Weiss 2011; Kilic et al. 2012; Kalirai 2012). 50 years ago. The first attempt to decipher the history of the formation of Whilst the hypothesis of strict ordering of the orbital prop- erties with metallicity in the stellar halo is not supported by the ⋆ This paper is dedicated to the memory of Professor Donald Lynden-Bell data, undeniably, a large fraction of the halo stars are metal-poor † E-mail:[email protected] and do move on eccentric orbits. In spherical polar coordinates one c 0000 The Authors 2 Vasily A. Belokurov et al ties may be drastically different due to the highly stochastic nature 10 10 of the stellar halo accretion (see e.g. Cooper et al. 2010). Indeed, in numerical simulations of stellar halo formation, the anisotropy 5 5 behavior differs markedly from that of the DM one. Already in the 0 0 central parts of the Galaxy, β quickly reaches ∼ 0.5 and rises fur- Z, kpc Z, kpc ther to β ∼ 0.8 at 100 kpc, showing a much smoother monotonic -5 -5 behavior all the way to the virial radius (see e.g. Abadi et al. 2006; Sales et al. 2007; Rashkov et al. 2013; Loebman et al. 2017). -10 -10 In this Paper, we explore the behavior of the stellar halo ve- 0 5 10 15 20 25 -3 -2 -1 0 locity ellipsoid as a function of metallicity and distance above the R, kpc [Fe/H] Galactic plane. Our sample is based on the SDSS DR9 spectro- scopic dataset and consists of Main Sequence stars with distances between < 1 and ∼ 10 kpc from the Sun. The stars included Figure 1. Galacto-centric view of the SDSS-Gaia sample. Left: Logarithm in the analysis overlap widely with selections considered by sev- of the stellar density in cylindrical R, z coordinates. Right: Logarithm of stellar density in the plane of [Fe/H] and Galactic height z. Horizontal dot- eral groups previously (see Smith et al. 2009; Bond et al. 2010; ted lines give the range of heights considered in the velocity ellipsoid anal- Loebman et al. 2014; Evans et al. 2016). However, thanks to the ysis. Gaia DR1, the proper motions of these stars are now measured with an improved accuracy. Section 2 gives the details of the stellar sam- ple under consideration, as well as the model used to characterize the stellar halo behavior. The implications of our findings and the can characterize the shape of the stellar halo’s velocity ellipsoid 2 2 comparison to the numerical simulations of the stellar halo forma- σθ +σφ by one number, the anisotropy parameter: β = 1 − 2 , where tion are presented in Section 3. 2σr β = −∞ for circular orbits and β = 1 for radial ones. In the So- lar neighborhood, the halo’s velocity ellipsoid is evidently radially biased. For example, Chiba & Yoshii (1998) obtained β = 0.52 for a small sample of local halo red giants and RR Lyrae ob- 2 DATA AND MODELLING served by the Hipparcos space mission. Using the SDSS Stripe 82 2.1 Main Sequence stars in SDSS-Gaia proper motions, and thus going deeper, Smith et al. (2009) mea- sured β = 0.69 using ∼ 2, 000 nearby sub-dwarfs. Combin- We select Main Sequence stars from the SDSS DR9 spectroscopic ing the SDSS observations with the digitized photographic plate sample (Ahn et al. 2012) by using the following cuts: |b| > 10◦, −1 measurements, Bond et al. (2010) increased the stellar halo sam- Ag < 0.5 mag, σRV < 50 kms , S/N> 10, 3.5 < log(g) < 5, ple further and derived β = 0.67. For slightly larger volumes 0.2 < g − r < 0.8, 0.2 < g − i < 2, 4500 K< Teff < 8000 probed with somewhat more luminous tracers, similar values of K and 15 <r < 19.5. All magnitudes are de-reddened using β ∼ 0.5 were obtained (see e.g. Deason et al. 2012; Kafle et al. dust maps of Schlegel et al. (1998). A total of 192,536 stars sur- 2012). Unfortunately, beyond 15-20 kpc from the Sun, the be- vives the above cuts. We estimate the stars’ distances using equa- haviour of the anisotropy parameter is yet to be robustly deter- tions (A2), (A3) and (A7) in Ivezi´cet al. (2008). The proper mo- mined. While there have been several attempts to tease β out of tions are obtained using a crossmatch between the SDSS and Gaia the line-of-sight velocity measurements alone (see e.g. Sirko et al. catalogs. The SDSS catalog has been astrometrically recalibrated 2004; Williams & Evans 2015), Hattori et al. (2017) have shown using the GaiaSource positions in order to correct both small and that these claims need to be taken with a pinch of salt. Curiously, large scale systematic astrometric errors. The resulting SDSS-Gaia the only distant stellar halo anisotropy estimate which actually proper motion catalog covers the entirety of the high latitude SDSS relies on the proper motion measurements (with HST) reports a sky with baselines between 6 and 18 years.
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